Gadsden Purchase: Now that's what I call corruption!

I've just started skimming Allan Nevin's Ordeal of the Union - he starts with a leisurely tour d'horizon of the US around 1848, the names mostly a blur - but I'm struck by his section (Vol 1 p165ff) on

Industrial pressure groups, contractors, lobbyists, claim-agents and other wire-pullers [who] infested Washington in hundreds, spending money lavishly, and creating an atmosphere in which suspicion of corruption often grew to certainty.

The treaties with Mexico were, it seems, a particularly fruitful source of enrichment for on-the-ball legislators (emphasis mine):

In 1854 the Senate Foreign Relations Committee was discussing the Gadsden Treaty. "Clayton says," wrote Edward Everett, "that there has been the most frightful corruption in reference to the Garay grant in connection with the Gadsden Treaty. Sixty-seven per cent of it owned by members of the Senate."

Mass production!

That SFRC report is, of course, nowhere to be seen online. There is an etextInversiones, especulación y diplomacia: Las relaciones entre México y los Estados Unidos durante la dictadura santannista which looks promising. (The Gadsden Purchase was indeed one of Santa Anna's shenanigans.)

There is also a paper (PDF) Visión del istmo de Tehuantepec durante la guerra con México e inmediatamente después that mentions Garay. Not much else online.

Is 67% a record? What proportion of Congress benefited from the Crédit Mobilier scam, I wonder?